function Hu = ZermeloGrad(tLa, La, tu, u, ts, s)
x1 = interp1(ts, s(:,1), tu);
x2 = interp1(ts, s(:,2), tu);
la1 = interp1(tLa, La(:,1), tu);
la2 = interp1(tLa, La(:,2), tu);

Hu = la2.*cos(u)-la1.*sin(u);


